TP-AGB Stars to Date High-Redshift Galaxies with the Spitzer Space Telescope

Abstract

We present new stellar population models that include the contribution of the Thermally Pulsing Asymptotic Giant Branch (TP-AGB) phase also in the synthetic spectral energy distribution (SED). The TP-AGB phase is essential for a correct modeling of intermediate-age (0.2 $\lesssim$ t/Gyr $\lesssim$ 1÷2) stellar populations, because it provides ~ 40% of the bolometric contribution, and up to ~ 80% of that in the K-band. These models are obtained by coupling the energetic of the TP-AGB phase as calibrated with data of Magellanic Clouds star clusters [9], with empirical spectra of TP-AGB stars [3]. Now that the Spitzer Space Telescope (SST) allows the sigth of the rest-frame IR at high redshifts, these models provide the opportunity to use the TP-AGB phase as an age indicator also for high-redshift stellar populations. Here we focus on redshift ~ 3 and provide predictions of the colours of various galaxy models as will be measured by means of the IRAC imaging instrument on board the SST. We find a sizable magnitude difference between TP-AGB-dominated high-redshift stellar populations and those being older or younger. The first releases of GOODS data should allow a check of these predictions.

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